1. Field of the Invention
The present invention relates generally to imaging devices, and particularly to a imaging device having a transfer electrode pattern.
2. Description of the Related Art
Various kinds of CCD (charge-coupled device) imaging devices are proposed so far, in which a charge transfer portion is formed on a semiconductor substrate, transfer electrodes are disposed on the upper layer of the charge transfer portion through an insulating layer, signal electric charges being transferred by the charge transfer portion with application of drive pulses to the transfer electrodes. As the arrangement pattern of the transfer electrodes on the CCD imaging device, there are known an arrangement pattern having a multilayer structure in which the transfer electrodes are located such that the adjacent transfer electrodes are partly overlapping with each other and an arrangement pattern having a single layer structure in which the transfer electrodes are located such that the adjacent transfer electrodes are not overlapping with each other.
FIG. 1 of the accompanying drawings, for example, is a plan view showing a main portion of an image pickup region of a CCD imaging device including transfer electrodes with single layer structures and an electrode pattern thereof. FIG. 2 is a cross-sectional view taken along the line A-A of the CCD imaging device shown in FIG. 1. The illustrated examples show transfer electrodes for use with a vertical transfer register by which signal electric charges, received at light-receiving portions of the CCD imaging device, are transferred in the vertical direction.
As shown in FIG. 2, a light-receiving portion and a charge transfer portion (not shown) are provided within a silicon substrate 101, and transfer electrodes 107 and 108 made of a suitable material such as polycrystalline silicon are provided on the upper layer of the silicon substrate 110 through an insulating film 111 made of a suitable material such as silicon oxide. The transfer electrode 108 may serve as a read electrode to which a read voltage to read signal electric charges from the light-receiving portion to the charge transfer portion, that is, a gate electrode of a read gate portion as well.
As shown in FIG. 1, the transfer electrodes 107 and 108 are formed with patterns so as to avoid light-receiving portions 102 of the CCD imaging device. The transfer electrode portions 107 and 108 are located close to each other in the portion in which they are located along charge transfer portions 103. Also, the transfer electrodes 107 and 108 are located between the vertically-adjacent light-receiving portions (pixels) 102 and 102 so as to connect the adjoining vertical transfer registers 103 and 103.
On the other hand, as a method of transferring signal electric charges read out to the vertical transfer registers in the CCD imaging device, there is known a technology for forming a shunt wiring structure by which a shunt wiring made of a suitable thin sheet material having a small sheet resistance, such as aluminum (Al) and tungsten (W), is joined to the transfer electrode.
Cited Patent Reference 1: Official Gazette of Japanese laid-open patent application No. 2002-158925
Cited Patent Reference 2: Official Gazette of Japanese laid-open patent application No. 2003-7997
In the CCD imaging device shown in FIG. 1, when signal electric charges generated by light incident on the light-receiving portion 102 are read out to the vertical transfer register 103, a read voltage (a high-level pulse of three-value pulses) V1 is applied to the transfer electrode 108 that serves as the read electrode as well. Then, with application of this read voltage V1, the signal electric charges are read out from the light-receiving portion 102 to the vertical transfer register 103.
However, as mentioned above, since the transfer electrode 108, which serves as the read electrode as well, is extended between the light-receiving portions (pixels) 102 adjoining in the vertical direction, the read voltage V1 is also applied to this extended portion of the transfer electrode 108. Although a channel stop layer for separating the pixels is formed between the light-receiving portions (pixels) 102 and 102 which are adjoining each other in the vertical direction, a potential barrier of the channel stop layer formed beneath the transfer electrode 108 is broken (modulated) with application of this read voltage V1 so that signal electric charges are also transferred from the light-receiving portions 102 and 102 to the vertical direction. As a result, a problem arises, in which color mixture occurs in the vertical direction between the light-receiving portions 102 and 102 which are adjoining each other in the vertical direction. Specifically, a phenomenon occurs, in which a part of signal electric charges, which should be read out to the vertical transfer register 103, is leaked to other light-receiving portions (pixels) 102 and 102 which are adjoining each other in the vertical direction. In particular, when a gap between the light-receiving portions (pixels) 102 and 102 which are adjoining each other in the vertical direction is reduced or a width of the channel stop region is reduced in order to microminiaturize the pixel in size, a problem of color mixture tends to occur more easily.
In order to prevent the above-mentioned color mixture, the potential barrier at the channel stop layer can be strengthened by increasing an impurity concentration of the channel stop layer formed between the pixels adjoining in the vertical direction or by increasing the depth of the channel stop layer with implantation of ions in a multiple stage fashion. However, there arises another problem, in which the region of the light-receiving portion 102 is reduced when impurities are diffused in the lateral direction of the channel stop layer by an annealing process for activating the ion-implanted channel stop layer and in which the number of manufacturing processes is increased with addition of the ion implantation process.